At least you're ready to give a small share of blame to Harriet. I do lay greater share of blame on Emma.
IMO it's 60:40 as of now. :)
I think Harriet was already under the influence of this 'high/low' thing with all that gossip going on at Goddard's.
After refusing the proposal Harriet comments;
"I think Mrs. Goddard would be very much surprized if she knew what had happened. I am sure Miss Nash would -- for Miss Nash thinks her own sister very well married, and it is only a linen-draper."
Stepping stone - what I am picturing is Harriet using the opportunities provided readily by Emma to get into higher society.
So, yes, its not Emma, but the opportunities.
I don't agree with the Darcy/Bingley comparison.
JA isn't repeating herself. She has given us a character from a class which no other novel has, and the same with another character who is illegitimate.
In addition to the origin of the characters the background of the whole situation is also different. Emma and Harriet have just met a couple of months ago compared to a friendship of several years.
Bingley owes nothing and no loyalty to Jane whereas Harriet has been welcomed with open arms by the Martin family *and* has given signs of encouragement to Robert Martin, which Jane didn't.
I'm sorry, but I have to disagree regarding Harriets loyalty towards the Martins. She's not with Emma 24 hours, and has enough time to think and not be pushed over.....and feel....if she wanted to.
Even Emma credited her with this when she invited Harriet to stay often at Hartfield after refusing the proposal.
Even Emma was deceived there, as Harriet didn't reflect earlier, nor was she going to do now.
More than Emma's it's Harriet's behaviour towards the Martins that I feel very bad about.